I. Field of the Invention
The present invention relates to an instrument for collecting blood, particularly, arterial blood.
II. Description of the Prior Art
In order to examine the pulmonary functioning and acid-base balance of a human body, blood tests called blood gas analysis are performed. Such tests include measurement of the dissolved oxygen and carbon dioxide content of arterial blood; measurement of the oxygen saturation or the pH of arterial blood; and quantitative analysis of the blood content of electrolytes such as HCO.sub.3.sup.-, Na.sup.+, K.sup.+, or Cl.sup.-.
In such blood gas analysis, if air is introduced into a blood sample to be tested, measurement precision is adversely affected. Therefore, air must not be introduced into the blood sample after collection.
For this purpose, it has been proposed to collect blood after drawing a heparin solution into a blood collection space of a blood collector, or to use a blood collection container in which a heparin solution is prefilled (Japanese Utility Model Nos. 53-49268, 52-42064). When such measures are taken, the dead space in the blood collection container and in a blood collection cannula is filled with the heparin solution to prevent any air from remaining therein. A heparin solution which may be generally used has a heparin concentration of about 1,000 units/ml. A diluent may be water, physiological saline solution or the like. The heparin solution thus prepared is mixed with the collected blood to dilute it. However, the results of blood gas analysis vary, due to variations in the degree of dilution of blood by the heparin solution, so that correct blood gas measurements may not be performed.
In view of this, it has also been proposed in U.S. Pat. No. 4,257,426 to introduce dry heparin as an anticoagulant into a blood collection space so that the problem of variations in the degree of dilution of blood by the heparin solution may be prevented, and to incorporate a separate mechanism in the blood collector for removing air and for providing an air-tight seal between the blood and ambient air. The above patent discloses a blood collector in which a dry heparin mass is placed in a blood collection space thereof. However, depending on the tilt angle of the blood collector, the heparin mass may not be situated in the vicinity of the blood inlet port at the closed end of an outer cylinder. Therefore, if blood pressure is high, initially introduced blood may not be brought into contact with the heparin mass, so that the heparin mass may not serve its original purpose. In this case, air becomes trapped between the hub and the outer cylinder.